Controlling chilled state of a cargo

ABSTRACT

A method for operating a refrigeration system for a container for refrigerating chilled cargo includes providing a refrigeration system including a compressor and an evaporator fan associated with an evaporator. The method also includes determining the temperature of supply air and the temperature of return air. The method further includes determining one of a requirement for heating and a requirement for cooling based on the temperature of the return air and the temperature of the supply air. The method additionally includes activating the evaporator fan when a requirement for heating is determined and increasing the speed of the evaporator fan when increased heating is determined. The method also includes activating the compressor and the evaporator fan when a requirement for cooling is determined and increasing the power supplied to the compressor and maintaining the evaporator fan at a first speed when increased cooling is determined.

FIELD OF THE INVENTION

This invention relates to climate control in cargo containers and todevices for controlling the climate in cargo containers. In particularthe invention relates to refrigeration systems for use in cargocontainers and methods for operating such systems for controlling acargo in a chilled state.

BACKGROUND OF THE INVENTION

The present invention relates to transporting and storing temperaturesensitive cargo over long periods of time using a controlled climate inthe space where the cargo is loaded. Climate control includescontrolling the temperature of the cargo within a certain acceptablerange. Controlling the temperature includes bringing the temperature ofthe cargo into the acceptable range (by refrigerating or heating) andmaintaining the temperature within that range. Climate control may alsoinclude controlling other parameters such as humidity and composition ofthe atmosphere.

Refrigeration is the process of removing heat from an enclosed space, orfrom a substance, and moving it to a place where it is unobjectionable.The primary purpose of refrigeration is lowering the temperature of theenclosed space or substance and then maintaining that lower temperature.

One commonly used refrigeration technique is the vapor-compressioncycle. The vapor-compression cycle is used in most householdrefrigerators as well as in many large commercial and industrialrefrigeration systems.

A refrigerated container or reefer is a shipping container used inintermodal freight transport, including rail, ship and truck, where thecargo is refrigerated (chilled or frozen) for the transportation oftemperature sensitive cargo. A reefer will usually have an integralrefrigeration unit.

The reliability of the refrigeration unit is of paramount importance.The temperature of temperature sensitive cargo should be kept withinpredefined limits. Some cargo must be maintained frozen, and thetemperature of any part of the frozen cargo must be kept below apredefined freezing temperature which depends on the cargo, e.g. below−18 degrees C. or lower, while other cargo, in particular commoditiessuch as fresh meat, fresh fruit and vegetables, should be kept chilledto stay fresh, but not frozen. For chilled fruit and vegetables there isa lowest acceptable temperature below which the commodity will begindegrading and loose its freshness. Such temperature is dependent uponthe type of fruit.

SUMMARY OF THE INVENTION

In one embodiment, the invention provides a method and a system forcontrolling the temperature of a cargo in a chilled state. In chilledstate the cargo is to be maintained at or near a set-point temperatureT_(SP) and for certain commodities and not below the set-pointtemperature. Whether heating or refrigeration is required the inventionensures a minimal consumption of energy with proper use of forced aircirculation, refrigeration and heating depending on the actualrequirement.

In one embodiment, the invention provides a method for operating arefrigeration system for a container for refrigerating chilled cargo.The method includes providing a refrigeration system including acompressor, a condenser, and an evaporator connected in series, anevaporator fan associated with the evaporator, and a heater. The methodalso includes determining the temperature of the supply air dischargedinto the container and the return air from the container, anddetermining one of a requirement for heating and a requirement forcooling based on the temperature of the return air and the temperatureof the supply air. The method further includes activating the evaporatorfan when a requirement for heating is determined and increasing thespeed of the evaporator fan when increased heating is determined, andactivating the compressor and the evaporator fan when a requirement forcooling is determined and increasing the power supplied to thecompressor and maintaining the evaporator fan at a first speed whenincreased cooling is determined.

Another embodiment of the invention is directed to a refrigerationsystem for a container for refrigerating chilled cargo. The systemincludes a compressor, a condenser, and an evaporator connected inseries. The system also includes a heater and an evaporator fanassociated with the evaporator, where the evaporator fan is operable todischarge supply air to the container and to receive return air from thecontainer. The system further includes sensors configured to sense thetemperature of the supply air and the temperature of the return air. Thecontroller is programmed to determine one of a requirement for heatingand a requirement for cooling based on the temperature of the return airand the temperature of the supply air. Where the controller is furtherprogrammed to activate the evaporator fan when a requirement for heatingis determined and to increase the speed of the evaporator fan whenincreased heating is determined. Where the controller is programmed toactivate the compressor and the evaporator fan when a requirement forcooling is determined and to increase the power supplied to thecompressor and maintain the evaporator fan at a first speed whenincreased cooling is determined.

Other aspects of the invention will become apparent by consideration ofthe detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically a refrigeration system according to theinvention.

FIG. 2 shows a refrigerated container with the refrigeration system inFIG. 1 installed.

FIG. 3 is a diagram illustrating the operation of the invention independence on the actual requirement for heating or refrigeration.

DETAILED DESCRIPTION OF THE INVENTION

Before any embodiments of the invention are explained in detail, it isto be understood that the invention is not limited in its application tothe details of construction and the arrangement of components set forthin the following description or illustrated in the following drawings.The invention is capable of other embodiments and of being practiced orof being carried out in various ways.

FIG. 1 is a simplified diagram of the basic components of a typicalone-stage vapor-compression refrigeration system 100 according to theinvention. In this cycle, a circulating refrigerant enters thecompressor 110 as a vapor. In the compressor the vapor is compressed andexits the compressor superheated. The superheated vapor travels throughthe condenser 120 which first cools and removes the superheat and thencondenses the vapor into a liquid by removing additional heat atconstant pressure and temperature. The liquid refrigerant goes throughan expansion valve 130 (also called a throttle valve) where its pressureabruptly decreases, causing flash evaporation and auto-refrigeration of,typically, less than half of the liquid. That results in a mixture ofliquid and vapor at a lower temperature and pressure. The coldliquid-vapor mixture then travels through the evaporator 140 coil ortubes and is completely vaporized by cooling the warm return air RAreturning from the refrigerated space being blown by an evaporator fan150 across the evaporator coil or tubes. The cool supply air SA is blowninto the refrigerated space. The resulting refrigerant vapor returns tothe compressor inlet to complete the thermodynamic cycle. A condenserfan 160 removes condensation heat from the condenser 120. A controller170 controls the operation of the refrigeration system and itsindividual components.

During operation water vapor will condensate on the evaporator 140 andform a layer of ice which will degrade the efficiency of the evaporator.The ice is removed in defrosting cycles where the compressor 110 and theevaporator fan 150 are inactivated, and a heater 180 is activated whichwill heat the evaporator 140. A temperature sensor 190 senses thetemperature of the evaporator 140 and when it has been determined, basedon the sensed evaporator temperature, that the ice is melted, thecompressor 110 is again activated. When the temperature of theevaporator is sufficiently low the evaporator fan 150 is activated andthe refrigeration system is in operation again.

The refrigeration system 100 can have one or more evaporator fans 150.The power of the evaporator fan motors can be controlled in two or moresteps or continuously by the controller 170. For simplicity, only highspeed operation and low speed operation are described, but the personhaving ordinary skill in the art will understand that the describedmethod applies in general to motors with controllable speed.

FIG. 2 shows schematically a portion of a refrigerated container 200loaded with cargo 210 to be refrigerated. The container 200 has arefrigeration system 100 installed in one end, and the container hasdoors (not shown) in the opposite end for loading and unloading thecargo 210. The evaporator fan or fans 150 of the refrigeration system100 blow refrigerated supply air SA into the container where itcirculates around the cargo 210 and returns as return air RA to therefrigeration system 100.

The energy required for circulating the air in the container isultimately dissipated as heat in the container due to friction.Depending on whether the evaporator fan 150 is operated in a low speedmode or in a high speed mode it delivers from a few hundred watts up toa few kilowatts (kW) which is dissipated as heat in the container. Thisenergy adds to the energy that enters the container from the ambient andthe heat that is generated by the cargo itself, all of which must beremoved by the refrigeration system. Assuming efficiencies of 100% ofboth the evaporator fan and the refrigeration system, for each kWconsumed by the evaporator fan another kW will be consumed by therefrigeration system.

FIG. 3 illustrates the operation of the invention in dependence on theactual requirement for heating or refrigeration. Based on observedtemperature T_(RA) of the return air RA and the temperature T_(SA) ofthe supply air SA and the difference T_(RA)−T_(SA) of the twotemperatures the requirement for heating or refrigeration is calculated.

When the observed temperature difference T_(RA)−T_(SA) indicates thatneither heating nor refrigeration is required, then none of thecompressor, the evaporator fan and the heater is operated, since thereis nothing to correct. However, at predetermined intervals theevaporator fan is activated to circulate air in the container and todraw the return air stream past the return air temperature sensor tomeasure its temperature to determine whether heating or refrigeration isneeded.

When a requirement for moderate heating is determined as in interval H₁,the evaporator fan is activated to circulate air in the container at arate where the friction heat generated by the air flow satisfies theneed for heating. This is possible with fan motors with continuouslyvariable speed, and with other motors it can be obtained by pulse widthmodulation (PWM) of the electric power supplied to the motors. The speedof other (traditional) fan motors can be controlled by turning them onand off at relatively longer intervals resulting in the correct meanvalue of the motor speed.

At higher requirements for heating than satisfied by the evaporator fanalone, as in interval H₂, the evaporator fan is operated at its fullcapacity and supplemented by the heater 180. The heater power isadjusted so that the friction heat from the air flow plus the heatgenerated by the heater satisfy the need for heating. The electric powersupplied to the heater can be varied e.g. by pulse width modulating thepower.

When a requirement for moderate refrigeration is determined as ininterval R₁, the compressor 110 is activated and the evaporator fan isactivated to circulate the air in the container and pass it through theevaporator coil to be refrigerated. The circulation of air results infriction heat dissipated in the container which adds to the energy to beremoved by refrigeration. The evaporator fan motor is therefore operatedat a low speed which is sufficient to circulate the air so as to meetthe requirement for refrigeration and to dissipate as little frictionheat as possible. Variations in the refrigeration requirement areaccommodated for by regulating the compressor power.

Letter A in FIG. 3 indicates a requirement for refrigeration determinedby the set-point temperature T_(SP), by the ambient conditions outsideof the container, which leaks heat energy into the container, and by theheat energy generated by the cargo, all of which are known or can bedetermined by measurements or observations or possibly estimated. Atrequirements for refrigeration higher than the requirement value A it isnecessary to circulate the air in the container at a high rate andcorrespondingly operate the evaporation fan at a high speed. Variationsin the refrigeration requirement are accommodated for by regulating thecompressor power.

The change in fan speed at requirement A affects the air flow throughthe evaporator, and therefore the compressor power is adjustedaccordingly.

The interval around the requirement value A is relatively narrow and thespeed of the evaporator fan motor may be varied continuously over thisinterval.

Various features and advantages of the invention are set forth in thefollowing claims.

What is claimed is:
 1. A method for operating a refrigeration system for a container for refrigerating chilled cargo, the method comprising: providing a refrigeration system including a compressor, a condenser, and an evaporator connected in series, an evaporator fan associated with the evaporator, and a heater, the refrigeration system operable to discharge supply air to the container and to receive return air from the container; determining the temperature of the supply air; determining the temperature of the return air; determining one of a requirement for heating and a requirement for cooling based on the temperature of the return air and the temperature of the supply air; activating the evaporator fan when a requirement for heating is determined and increasing the speed of the evaporator fan when increased heating is determined; and activating the compressor and the evaporator fan when a requirement for cooling is determined and increasing the power supplied to the compressor and maintaining the evaporator fan at a first speed when increased cooling is determined.
 2. The method of claim 1, wherein determining the requirement for heating or refrigeration is performed at predetermined time intervals.
 3. The method of claim 1, further comprising deactivating the compressor and the evaporator fan when there is no requirement determined for at least one of heating and cooling.
 4. The method of claim 1, further comprising activating the heater to heat the supply air when the requirement for heating is higher than can be satisfied by the evaporator fan alone, and increasing the power supplied to the heater when increased heating is determined.
 5. The method of claim 1, increasing the speed of the evaporator fan to a second speed faster than the first and increasing the power supplied to the compressor when increased cooling is determined.
 6. The method of claim 5, wherein the time at which the evaporator fan speed is increased from the first speed to the second speed is determined by the set-point temperature, the ambient conditions, and by the heat energy generated by the cargo within the container.
 7. The method of claim 1, wherein the evaporator fan is operated by a controllable evaporator fan motor.
 8. The method of claim 7, wherein the evaporator fan motor has a continuously variable speed.
 9. The method of claim 8, wherein the evaporator fan motor uses a pulse width modulation (PWM) of an electric power supplied to the motor.
 10. The method of claim 1, wherein determining one of a requirement for heating and a requirement for cooling includes determining one of a requirement for heating and a requirement for cooling based on a difference between the return air temperature and the supply air temperature.
 11. A method for operating a refrigeration system for a container for refrigerating chilled cargo, the method comprising: providing a refrigeration system including a compressor, a condenser, and an evaporator connected in series, an evaporator fan associated with the evaporator, and a heater, the refrigeration system operable to discharge supply air to the container and to receive return air from the container; determining the temperature of the supply air; determining the temperature of the return air; determining at predetermined time intervals one of a requirement for heating and a requirement for cooling based on a difference between the temperature of the return air and the temperature of the supply air; activating the evaporator fan when a requirement for heating is determined and increasing the speed of the evaporator fan when increased heating is determined; activating the compressor and the evaporator fan when a requirement for cooling is determined and increasing the power supplied to the compressor and maintaining the evaporator fan at a first speed when increased cooling is determined; deactivating the compressor and the evaporator fan when there is no requirement determined for at least one of heating and cooling; activating the heater to heat the supply air when the requirement for heating is higher than can be satisfied by the evaporator fan alone, and increasing the power supplied to the heater when increased heating is determined; and increasing the speed of the evaporator fan to a second speed faster than the first and increasing the power supplied to the compressor when increased cooling is determined. 